Airtight silicon lens inversion hot reflow process method and jig

Abstract

The application discloses a closed silicon lens inversion heat reflow process method and jig and belongs to the technical field of silicon lens manufacturing. The method comprises the following steps: preheating an oven to 75-85 DEG C; loading a wafer into a jig in an effective surface downward manner, and the wafer is only in point contact or line contact with the jig through an edge non-effective surface; placing the jig into the oven after covering the jig to form a sealed chamber; filling protective gas to discharge air and keeping normal pressure; increasing the temperature to 140-200 DEG C at a rate of 3-7 DEG C / min and keeping the temperature for 5-10 min; and taking out and cooling to perform the next operation. The jig comprises a base and an upper cover, the inner edge of the base is provided with annularly arranged support columns, the support columns are provided with point contact or line contact support structures, and the support surface of the support face is equal in distance to the uppermost part and the lowermost part of the sealed chamber. The application effectively improves the surface type, the curvature radius and the height uniformity of the microlens array through multiple means such as wafer inversion, closed jig air flow isolation, point / line contact support and normal pressure environment, and is suitable for large-scale mass production.

Description

Technical Field

[0001] This invention relates to the field of silicon lens manufacturing technology, and more specifically to a closed-type silicon lens inverted hot reflow process method and fixture. Background Technology

[0002] With the rapid development of optoelectronic systems towards miniaturization, high-density integration, high infrared transmittance, and wafer-level packaging, downstream demand in fields such as optical communication CPO, automotive LiDAR, infrared thermal imaging, miniature medical endoscopes, and 3D sensing continues to surge, driving silicon-based microlenses to become core optical components in high-end optoelectronic equipment. Compared to traditional glass lenses, silicon lenses possess significant advantages such as excellent infrared transmittance, good thermal stability, high thermal expansion matching with silicon-based chips, and compatibility with semiconductor wafer-level mass production. They effectively address the pain points of traditional optical devices, such as large size, high integration difficulty, and poor consistency, making domestic substitution and high-end capacity upgrades urgently needed in the industry. Currently, the mainstream mass production solution in the industry is a combination of photoresist thermal reflow and dry etching topology transfer process. This process is compatible with standard semiconductor production lines and can achieve simultaneous forming of the entire wafer microlens array. It features low mass production cost, high surface finish, and good array uniformity, making it the core technology for the mass production of spherical silicon microlenses at present.

[0003] However, the existing traditional hot reflow process still has obvious technical shortcomings. The range of lens curvature and sag control is limited, making it difficult to fabricate large numerical aperture and aspherical structure lenses. At the same time, the process parameters are sensitive to the forming effect. Inadequate temperature control and inability to manage airflow disturbances can easily lead to problems such as lens distortion, array coaxiality deviation, and insufficient uniformity of wafer surface shape. These issues restrict the industrial application of high-precision and high-performance silicon microlenses. Therefore, it is urgent to optimize and improve the hot reflow forming technology to break through the existing process bottlenecks. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a closed-loop silicon lens inverted hot reflow process method and fixture. Through multiple means such as wafer inversion, closed-loop fixture to isolate airflow, point / line contact support, and normal pressure environment, the surface shape, radius of curvature, and uniformity of the microlens array are effectively improved, making it suitable for mass production.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A closed-loop silicon lens inverted hot reflow process includes the following steps: S1. Preheat the oven to 75-85℃; S2. The wafer is placed upside down into the fixture with the effective side facing down, and the wafer only contacts the fixture through the edge non-effective side; S3. After closing the fixture to form a sealed chamber, place it in the oven; S4. Fill the oven with protective gas, expel the air, and maintain a normal pressure environment; S5. The oven is heated at a rate of 3-7℃ / min for 25-40 minutes, raising the temperature from 75-85℃ to 140-200℃, and then held at 140-200℃ for 5-10 minutes. S6. After the hot reflow is completed, remove the fixture and allow it to cool for 10-20 minutes before proceeding to the next round of work.

[0006] Furthermore, in step S2, after the wafer is placed into the fixture, the distance between the wafer and the top of the inner sealed chamber of the fixture and the bottom of the inner sealed chamber of the fixture are equal.

[0007] Furthermore, the protective gas in step S4 is nitrogen or argon.

[0008] Furthermore, in step S5, the heating rate of the oven is 5℃ / min, the heating time is 30 minutes, the target temperature is 160-180℃, and the holding time is 8 minutes.

[0009] A fixture for a sealed silicon lens inverted hot reflow process includes a base and a top cover. The top cover can be closed onto the base to form a sealed chamber inside the base and the top cover. A plurality of annularly arranged support columns are provided at the inner edge of the base. Support structures are provided on the support columns. The support structures are point contact or line contact structures. The distance between the support surface of the support structure and the uppermost and lowermost parts of the sealed chamber is equal.

[0010] Furthermore, the support structure is a point contact structure, which includes support pins disposed on support pillars. When the wafer is placed, a plurality of support pins contact and support the wafer.

[0011] Furthermore, the support structure is a line contact structure, and a placement platform is provided on the side of the support column facing the center. The center of the placement platform is arched into an arc shape, and the wafer can be placed on the placement platform and make line contact.

[0012] Furthermore, the base and the top cover are directly equipped with a locking structure.

[0013] Furthermore, the base and the top cover are directly equipped with sealing rings.

[0014] Furthermore, the base and top cover are made of high-temperature resistant metal or high-temperature resistant ceramic materials.

[0015] Compared with the prior art, the beneficial effects of the present invention are: 1. Significantly improves the uniformity of the microlens array: Through multiple methods such as wafer inversion, edge point / line contact support, and wafer positioning at the geometric center of the sealed chamber, the influence of solid-state heat conduction and temperature field asymmetry on the in-plane temperature distribution of the wafer is minimized. The wafer is heated uniformly mainly through thermal radiation and thermal convection, with minimal temperature differences between different regions within the wafer. This ensures that the thousands or even tens of thousands of microlenses on the entire wafer maintain a high degree of consistency in terms of surface shape, ROC, and sag.

[0016] 2. Effectively avoids "resin bursting" and surface skinning defects: The wafer is placed in an oven preheated to 75-85℃, allowing it to gradually adapt to the temperature environment and avoiding the thermal shock caused by directly placing a room-temperature wafer into a high-temperature environment. Combined with a gentle heating rate of 3-7℃ / min, this ensures that the temperature inside and outside the photoresist pillar rises and melts simultaneously, eliminating the "resin bursting" phenomenon. Solvents evaporate slowly under normal pressure, avoiding the problem of premature surface hardening and skinning under vacuum conditions.

[0017] 3. Eliminating the impact of airflow disturbance on molding quality: The sealed fixture isolates the wafer from the airflow environment of the external oven cavity, placing the wafer in a static gas atmosphere within the sealed chamber. Even if airflow disturbances caused by hot air circulation exist inside the oven, they will not directly affect the photoresist pillars on the wafer surface, completely eliminating the interference of airflow on the molten photoresist molding process.

[0018] 4. Balancing mass production and process flexibility: The oven-type heating method can simultaneously accommodate multiple fixtures for synchronous thermal reflow of multiple wafers, significantly increasing capacity compared to the hot plate method for single-wafer processing. Furthermore, after thermal reflow, only a 10-20 minute cooling time is required before starting the next batch, allowing for flexible batch switching, high equipment utilization, and suitability for large-scale mass production needs.

[0019] 5. Gravity-Assisted Spherical Formation Enhances Lens Fullness: The wafer is inverted with its effective surface facing down. The molten photoresist, in addition to its surface tension, is further subjected to the downward force of gravity. Both forces are aligned, driving the photoresist downwards to form a spherical shape. Compared to the upright position (where gravity and surface tension are opposite or perpendicular), the inverted position makes it easier to achieve a full spherical profile, especially beneficial for fabricating microlenses with large sag and large numerical aperture.

[0020] 6. Self-generated pressure effect within the sealed chamber improves surface uniformity: During wafer heating, the organic solvent vapors released by the photoresist are confined within the sealed chamber and cannot escape. As the temperature rises, the pressure within the sealed chamber naturally increases. This uniform static pressure acts on the surface of all microlenses, subjecting the molten photoresist to isotropic compressive stress during spherical formation. This helps eliminate local surface defects and further improves the uniformity of the on-chip lens surface shape, ROC, and sagitta.

[0021] 7. The fixture features a compact, durable, and easy-to-maintain structure: the base and top cover are quickly and easily closed via a snap-fit ​​mechanism; the sealing ring ensures airtightness; and the high-temperature resistant metal or ceramic material guarantees dimensional stability and corrosion resistance during long-term use. The point / line contact structure, such as the support pin or the arc-shaped placement platform, offers high machining precision and allows for quick replacement after wear, reducing maintenance costs and lifespan expenses. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort, wherein: Figure 1 A schematic diagram of the structure of a fixture used in a closed-type silicon lens inverted hot reflow process. Figure 1 ; Figure 2 An exploded view of a fixture used in a closed-loop silicon lens inverted hot reflow process. Figure 3 for Figure 2 Enlarged view of point A in the middle; Figure 4 A schematic diagram of the structure of a fixture used in a closed-type silicon lens inverted hot reflow process. Figure 2 .

[0023] The diagram is labeled as follows: 1. Base; 2. Top cover; 3. Wafer; 4. Support pillar; 5. Support structure. Detailed Implementation

[0024] In the description of this invention, it should be noted that the directional terms such as "center", "horizontal (X)", "longitudinal (Y)", "vertical (Z)", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", and "counterclockwise" indicate the orientation and positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. They should not be construed as limiting the specific protection scope of this invention.

[0025] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features. Thus, the use of "first" and "second" to define a feature may explicitly or implicitly include one or more of that feature. In the description of this invention, "several" or "a number" means two or more, unless otherwise explicitly specified.

[0026] A closed-loop silicon lens inverted hot reflow process includes the following steps: S1. Preheat the oven to 75-85℃; S2. The wafer 3 is placed upside down into the fixture with the effective side facing down, and the wafer 3 only contacts the fixture through the edge non-effective side; S3. After closing the fixture to form a sealed chamber, place it in the oven; S4. Fill the oven with protective gas, expel the air, and maintain a normal pressure environment; S5. The oven is heated at a rate of 3-7℃ / min for 25-40 minutes, raising the temperature from 75-85℃ to 140-200℃, and then held at 140-200℃ for 5-10 minutes. S6. After the hot reflow is completed, remove the fixture and allow it to cool for 10-20 minutes before proceeding to the next round of work.

[0027] Preferably, in step S2, after the wafer 3 is placed in the fixture, the distance between the wafer 3 and the topmost and bottommost parts of the sealed chamber inside the fixture are equal. Specifically, wafer 3 is positioned at the geometric center of the sealed chamber, making the sealed space on its upper and lower sides equal in volume. When the oven heats the sealed chamber as a whole, heat is transferred to wafer 3 through the sealed chamber wall by means of thermal radiation and internal gas convection. The symmetrical spatial structure on the upper and lower sides makes the heat transfer path length equal and the thermal resistance the same.

[0028] This design ensures that the thermal radiation intensity and thermal convection conditions received by both sides of wafer 3 are completely symmetrical, eliminating the temperature field differences caused by the spatial asymmetry of wafer 3. The temperature distribution across wafer 3 is more uniform, and the photoresist pillars are heated under consistent conditions during melting, thereby significantly improving the uniformity of the on-chip microlens array in terms of surface shape, radius of curvature (ROC), and sagitta.

[0029] Preferably, the protective gas in step S4 is nitrogen or argon; Specifically, nitrogen and argon are both chemically inert gases and do not react chemically with the photoresist at high temperatures. Continuously introducing protective gases during the reflow process expels and replaces oxygen in the sealed chamber and oven cavity, placing the wafer surface in an inert atmosphere. This effectively prevents oxidation and deterioration of the photoresist during high-temperature melting, avoiding the formation of impurities such as oxides or carbides on the surface, and ensuring the purity and optical transmittance of the microlens surface. Simultaneously, the melting and flow behavior of the photoresist is more stable and controllable under an inert atmosphere, which is beneficial for obtaining lens structures with high surface smoothness and uniform contours.

[0030] Preferably, in step S4, the environment is kept at normal pressure. The heat reflow is carried out at normal pressure. The gas molecule density in the sealed chamber is relatively high, and the residual organic solvent in the photoresist can gradually evaporate and escape at a slow rate, rather than being rapidly extracted under vacuum negative pressure.

[0031] In a vacuum environment, the rapid evaporation of the surface solvent can cause premature hardening and skinning of the colloid film, while the internal solvent remains trapped and unable to escape, ultimately resulting in lens shape distortion and poor consistency in ROC and sagitta. Under normal pressure, the solvent evaporation process is gentle and controllable, and the colloid softens uniformly throughout, avoiding the uneven state of "surface skinning - internal melting," thus ensuring the uniformity of the microlens' shape, ROC, and sagitta.

[0032] Preferably, in step S5, the heating rate of the oven is 5℃ / min, the heating time is 30 minutes, the target temperature is 160-180℃, and the holding time is 8 minutes. Specifically, the transformation of photoresist from a glassy state to a highly elastic state and then to a viscous flow state requires the molecular chain segments to acquire mobility and undergo long-range slip. A moderate heating rate (3-7℃ / min) ensures a small temperature gradient inside the photoresist column, allowing all parts to reach the glass transition temperature (Tg) simultaneously; the target temperature range of 140-200℃ covers the temperature range required for the photoresist to fully melt; and the holding time of 5-10 minutes provides sufficient time for molecular rearrangement and surface tension-driven spherical formation.

[0033] A heating rate that is too slow will reduce production efficiency, while a rate that is too fast may lead to a "bursting" phenomenon—the surface temperature of the photoresist column exceeds the Tg (thermal flux) while the interior has not yet reached it, resulting in asynchronous melting and structural collapse. The heating rate of this invention is optimized to ensure both heating efficiency and synchronized temperature rise throughout the photoresist column. A temperature range of 140-200℃ and a holding time of 5-10 minutes provide a sufficient reflow window, allowing the photoresist to fully spherize under the combined effects of surface tension and gravity, resulting in a full spherical profile.

[0034] A fixture for a sealed silicon lens inverted hot reflow process includes a base 1 and a top cover 2. The top cover 2 can cover the base 1 to form a sealed chamber inside the base 1 and the top cover 2. A plurality of annularly arranged support columns 4 are provided at the inner edge of the base 1. Support structures 5 are provided on the support columns 4. The support structures 5 are point contact or line contact structures. The distance between the support surface of the support structure 5 and the uppermost and lowermost parts of the sealed chamber is equal.

[0035] Preferably, the support structure 5 is a point contact structure, which includes support pins disposed on the support column 4. When the wafer 3 is placed, a plurality of support pins contact and support the wafer 3.

[0036] Preferably, the support structure 5 is a line contact structure, and a placement platform is provided on the side of the support column facing the center. The center of the placement platform is arched into an arc shape, and the wafer 3 can be placed on the placement platform and make line contact.

[0037] Specifically, the support pin contacts the non-effective surface of the wafer 3 edge with its tip, theoretically resulting in a contact area close to zero; the curved placement platform creates an extremely narrow contact line between the wafer 3 and the platform. Both designs significantly reduce the solid contact area between the wafer 3 and the fixture.

[0038] In the manufacturing process, solid-state heat conduction is one of the main factors causing temperature non-uniformity within the three surfaces of a wafer—the temperature at contact points is often higher than that of non-contact areas, creating a temperature gradient within the three surfaces. By employing point contact or line contact supports, the wafer is primarily heated through thermal radiation and convection within a sealed chamber, rather than through solid-state conduction, resulting in a more uniform temperature distribution within the three surfaces. Simultaneously, the contact surfaces are limited to the non-effective edges (i.e., areas not containing microlens patterns), ensuring that the effective surfaces are completely free from contact with any objects. This avoids the risk of damage to the microlens structure on the effective surfaces due to contact, guaranteeing the integrity and yield of the lens array.

[0039] Preferably, the base 1 and the upper cover 2 are directly provided with a locking structure; Specifically, the locking structure can be a snap-on structure or a threaded tightening locking structure.

[0040] Preferably, the base 1 and the upper cover 2 are directly provided with sealing rings; Specifically, the sealing ring undergoes elastic deformation after the base 1 and the top cover 2 are closed, filling the gap between them and forming an airtight sealing structure.

[0041] This ensures the airtightness of the sealing chamber throughout the entire hot reflow process, preventing gases (which may contain oxygen or particulate matter) from the external oven cavity from entering the sealing chamber and contaminating the wafer surface. Simultaneously, the solvent vapors released during photoresist heating in the sealed environment are confined within the sealing chamber, creating a localized saturated vapor pressure. This further suppresses excessively rapid solvent evaporation, which is beneficial for the uniformity of the microlens' surface shape, ROC, and sagitta.

[0042] Preferably, the base 1 and the top cover 2 are made of high-temperature resistant metal or high-temperature resistant ceramic materials. High-temperature resistant metals (such as stainless steel, Invar alloy, etc.) and high-temperature resistant ceramics (such as alumina, silicon nitride, etc.) have good dimensional stability and chemical inertness in the working temperature range of 140-200℃.

[0043] During repeated heating and cooling cycles, the fixture does not undergo thermal deformation or oxidation corrosion, maintaining precise structural dimensions and sealing performance over a long period. The high-temperature resistant metal and ceramic materials have moderate thermal conductivity, which can evenly transfer heat from the oven into the sealed chamber without causing localized overheating, ensuring the uniformity of the temperature field inside the sealed chamber and long-term process stability.

[0044] advantage: 1. Significantly improves the uniformity of the microlens array: Through multiple methods such as wafer 3 inversion, edge point / line contact support, and positioning wafer 3 at the geometric center of the sealed chamber, the influence of solid-state heat conduction and temperature field asymmetry on the in-plane temperature distribution of wafer 3 is minimized. Wafer 3 is heated uniformly mainly through thermal radiation and thermal convection, with minimal temperature differences between different regions within the wafer. This ensures that the thousands or even tens of thousands of microlenses on the entire wafer 3 maintain a high degree of consistency in surface shape, ROC, and sag.

[0045] 2. Effectively avoids "resin bursting" and surface skinning defects: Wafer 3 is placed in an oven preheated to 75-85℃, allowing it to gradually adapt to the temperature environment and avoiding the thermal shock caused by directly placing room-temperature wafer 3 into a high-temperature environment. Combined with a gradual heating rate of 3-7℃ / min, this ensures that the temperature inside and outside the photoresist pillar rises and melts synchronously, eliminating the "resin bursting" phenomenon. Solvent evaporation is gradual under normal pressure, avoiding the problem of premature surface hardening and skinning under vacuum conditions.

[0046] 3. Eliminating the impact of airflow disturbance on molding quality: The sealed fixture isolates wafer 3 from the airflow environment of the external oven cavity, and wafer 3 is in a static gas atmosphere in the sealed chamber. Even if there is airflow disturbance caused by hot air circulation inside the oven, it will not directly affect the photoresist pillars on the surface of wafer 3, completely eliminating the interference of airflow on the molten photoresist molding process.

[0047] 4. Balancing mass production and process flexibility: The oven-type heating method can simultaneously accommodate multiple fixtures for multi-wafer synchronous thermal reflow, significantly increasing capacity compared to the single-wafer hot plate method. Furthermore, after thermal reflow, only a 10-20 minute cooling time is required before starting the next batch, allowing for flexible batch switching, high equipment utilization, and suitability for large-scale mass production needs.

[0048] 5. Gravity-Assisted Spherical Formation Enhances Lens Fullness: Wafer 3 is inverted with its effective surface facing down. The molten photoresist, in addition to its surface tension, is subjected to the downward force of gravity. Both forces are aligned, jointly driving the photoresist downward to form a spherical shape. Compared to the upright method (where gravity and surface tension are opposite or perpendicular), the inverted method makes it easier to obtain a full spherical profile for the lens, which is especially beneficial for the fabrication of microlenses with large sag and large numerical aperture.

[0049] 6. Self-generated pressure effect within the sealed chamber improves surface uniformity: During wafer heating, the organic solvent vapors released by the photoresist are confined within the sealed chamber and cannot escape. As the temperature rises, the pressure within the sealed chamber naturally increases. This uniform static pressure acts on all microlens surfaces, subjecting the molten photoresist to isotropic compressive stress during spherical formation. This helps eliminate local surface defects and further improves the uniformity of the on-chip lens surface shape, ROC, and sagitta.

[0050] 7. The fixture features a compact, durable, and easy-to-maintain structure: the base 1 and the top cover 2 are quickly closed via a snap-fit ​​mechanism, making operation simple; the sealing ring ensures airtightness; and the high-temperature resistant metal or ceramic material guarantees dimensional stability and corrosion resistance during long-term use. The point / line contact structure, such as the support pin or the arc-shaped placement platform, offers high machining precision and allows for quick replacement after wear, reducing maintenance costs and lifespan expenses.

[0051] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method for a sealed silicon lens inverted hot reflow process, characterized in that, Includes the following steps: S1. Preheat the oven to 75-85℃; S2. The wafer is placed upside down into the fixture with the effective side facing down, and the wafer only contacts the fixture through the edge non-effective side; S3. After closing the fixture to form a sealed chamber, place it in the oven; S4. Fill the oven with protective gas, expel the air, and maintain a normal pressure environment; S5. The oven is heated at a rate of 3-7℃ / min for 25-40 minutes, raising the temperature from 75-85℃ to 140-200℃, and then held at 140-200℃ for 5-10 minutes. S6. After the hot reflow is completed, remove the fixture and allow it to cool for 10-20 minutes before proceeding to the next round of work.

2. The method for a sealed silicon lens inverted hot reflow process according to claim 1, characterized in that: In step S2, after the wafer is placed into the fixture, the distance between the wafer and the top of the inner sealed chamber of the fixture and the bottom of the inner sealed chamber of the fixture are equal.

3. The method for a sealed silicon lens inverted hot reflow process according to claim 1, characterized in that: The protective gas mentioned in step S4 is nitrogen or argon.

4. The method for a sealed silicon lens inverted hot reflow process according to claim 1, characterized in that: The heating rate of the oven in step S5 is 5℃ / min, the heating time is 30 minutes, the target temperature is 160-180℃, and the holding time is 8 minutes.

5. A fixture for use in the closed-type silicon lens inverted hot reflow process according to any one of claims 1-4, characterized in that: The device includes a base and a top cover. The top cover can be closed onto the base to form a sealed chamber inside the base and the top cover. Several ring-shaped support columns are provided at the inner edge of the base. Support structures are provided on the support columns. The support structures are point contact or line contact structures. The distance between the support surface of the support structure and the uppermost and lowermost parts of the sealed chamber is equal.

6. The fixture according to claim 5, characterized in that: The support structure is a point contact structure, which includes support pins. The support pins are disposed on the support pillars, and when the wafer is placed, a number of support pins contact and support the wafer.

7. The fixture according to claim 5, characterized in that: The support structure is a line contact structure. A placement platform is provided on the side of the support column facing the center. The center of the placement platform is arched into an arc shape, allowing the wafer to be placed on the placement platform and make line contact.

8. The fixture according to claim 5, characterized in that: The base and the top cover are directly equipped with a locking structure.

9. The fixture according to claim 5, characterized in that: The base and the top cover are directly equipped with sealing rings.

10. The fixture according to claim 5, characterized in that: The base and top cover are made of high-temperature resistant metal or high-temperature resistant ceramic materials.

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